1. Field of the Invention
The present invention generally relates to an image reading apparatus configured to read a document image by driving an optical system that projects image light of the document on an image sensor in a sub scanning direction. The present invention particularly relates to an image reading apparatus that includes a power saving power circuit and an imaging apparatus using such an image reading apparatus. For example, an image reading apparatus according to an embodiment of the present invention may be used in a document scanner, a digital copier, or a facsimile machine.
2. Description of the Related Art
In Japanese Laid-Open Patent Publication No. 2004-258386, a flat-bed document scanner is disclosed that is configured to irradiate light on an original document 101 placed on a translucent document table 102 by a panel lamp 103 that is included in a first carriage 105, reflect the reflected light of the document (image light) by a first mirror 104 in a direction parallel to the sub scanning direction, reverse the direction of the reflected image light in the sub scanning direction by second and third mirrors 106 and 107 included in a second carriage 108 that is driven in the same direction at a speed ½ the speed of the first carriage 105, and project the image light on a CCD 109 by a lens 106. According to this disclosure, the CCD 110 converts image light into image signals. When the open angle of an original document cover 112 is less than 40 degrees, the powers of a lamp and the CCD 110 are tuned on, and when the open angle of the original document cover is 15 degrees or lower, the original document size is determined based on the image signals of the CCD (FIG. 17). In other words, the original document size is determined using the image signals of the CCD 107 that is configured to read the original document image when the original document cover is closed.
Japanese Laid-Open Patent Publication No. 6-189075 discloses an image reading apparatus that is configured to move an original document size reading black plate 104 and a shading correction white reference plate 103 back and forth from a position facing a sheet-through window 112 arranged at the other side of an original document passage. According to this disclosure, a connected structure of the black plate 104 and the white reference plate 103 is driven in a sheet transferring direction by a drive solenoid and a return spring, and the white reference plate 103 is read at the position facing the window 112 after which the black plate 104 is moved to the position facing the window 112 and read in order to detect the original document size.
Japanese Laid-Open Patent Publication No. 2004-109639 discloses an image reading apparatus that is configured to drive a flat-bed first carriage 12 to a position of the a white reference plate 5 upon switching from a standby mode to a power saving mode, and read the white reference plate 5 and drive the first carriage to an original document size reading position Ph upon returning from the power saving mode to the standby mode.
Japanese Laid-Open Patent Publication No. 2004-320333 discloses a copier apparatus in which an image reading unit is configured to acquire a power saving duration time from a printer engine controller upon returning from a power saving mode to standby mode; perform an initialization process involving setting a parameter determining the digital conversion parameters for an image signal processing circuit that digitally converts an image signal of a solid image capturing device such as a CCD into appropriate image data and store the set parameter in a nonvolatile memory when the acquired duration time is longer than a predetermined time; and read the parameter set in a previous initialization process from the nonvolatile memory and set the read parameter to the image signal processing circuit when the elapsed time from the time of the previous initialization process is shorter than the predetermined time.
In the disclosures cited above, for example, a document size detection technique is used that involves determining a main scanning direction width of a document placed on a document scanner without using a document size detecting sensor and determining such a document width based on an image signal of a CCD that converts a document image into image signals. By using such a technique, the number of sensors that have to be included in the document scanner may be reduced to thereby realize cost reduction. However, there are drawbacks to using such a document size detection technique as is described below. When the above-described technique is used, the document size detection is performed after the document scanner is switched to a document reading enabled state. Specifically, homing and/or AGC (automatic gain control) are performed from the time the power is supplied to the document scanner to the time the document size is detected. Homing refers to a process of initializing sub scanning position information to sub scanning position information detected by a reference point sensor when a carriage that scans a document in the sub scanning direction is driven in the sub scanning direction to be detected by the reference point sensor. AGC refers to a process of driving a carriage to a reference white plate reading position, reading the reference white plate, and updating the image signal amplitude gain and other parameters of an image signal processing circuit that digitally converts an image signal of an image sensor such as a CCD into image data so that the read data takes an appropriate value.
It is noted that in a case where a power circuit having a power saving mode is used, greater power saving effects may be realized when the switching waiting time for switching to the power saving mode is reduced. The switching waiting time for switching to the power saving mode refers to a predetermined value representing a time period during which a standby mode (i.e., mode in which operations may be immediately started in response to a user command) is to be maintained without receiving any operations command from a user, for example. When the time period during which no user command is received exceeds this predetermined time period, the power circuit is switched from the standby mode to the power saving mode.
The power circuit is switched back to the standby mode from the power saving mode in response to the user operating on the image reading apparatus. However, in a case where the user raises a document cover, places a document on a contact glass, lowers the document cover, and pushes a start key, for example, if such user operations are performed in a short time period, the document cover may be closed before the document size is detected. That is, even if the power circuit is switched back to the standby mode from the energy saving mode in response to the opening of the document cover, time is required for performing homing or AGC of the carriage and driving the carriage to the document size reading position, and thereby, the document cover may be closed before the document size can be detected.